According to the United States Census Bureau, Barnwell has a total area of 8.0 square miles (20.7 km2), of which 7.8 square miles (20.3 km2) is land and 0.15 square miles (0.4 km2), or 1.86%, is water.[3]

As of the census[1] of 2000, there were 5,035 people, 2,035 households, and 1,353 families residing in the city. The population density was 659.5 people per square mile (254.8/km²). There were 2,304 housing units at an average density of 301.8 per square mile (116.6/km²). The racial makeup of the city was 49.81% White, 47.37% Black, 1.05% Asian, 0.40% Native American, 0.04% Pacific Islander, 0.34% from other races, and 0.99% from two or more races. Hispanic or Latino of any race were 0.62% of the population.

There were 2,035 households out of which 34.3% had children under the age of 18 living with them, 40.2% were married couples living together, 22.1% had a female householder with no husband present, and 33.5% were non-families. 30.2% of all households were made up of individuals and 12.5% had someone living alone who was 65 years of age or older. The average household size was 2.42 and the average family size was 3.01.

In the city the population was spread out with 28.4% under the age of 18, 8.9% from 18 to 24, 27.6% from 25 to 44, 20.3% from 45 to 64, and 14.8% who were 65 years of age or older. The median age was 35 years. For every 100 females there were 86.8 males. For every 100 females age 18 and over, there were 79.7 males.

The median income for a household in the city was $26,722, and the median income for a family was $37,841. Males had a median income of $35,039 versus $21,912 for females. The per capita income for the city was $17,709. placing it in the top third of the state. About 20.4% of families and 22.0% of the population were below the poverty line, including 30.5% of those under age 18 and 16.7% of those age 65 or over.

In 1785 the district of Winton County was formed out of Orangeburg District in order to create another judicial circuit. It was given its current name in 1798 when the county and its seat were named for Revolutionary War leader John Barnwell (1748–1800), who headed a militia in South Carolina. Barnwell County originally stretched from the Savannah River on the west almost to the Atlantic Ocean.

Barnwell gave generously to the Confederate cause; the most distinguished person was General Johnson Hagood, who was later governor of South Carolina. Soon after Hagood's election, one of his constituents asked him if he wished to be called "General" or "Governor". "Call me General," Hagood said, "I fought for that and begged for the other."[citation needed]

Several towns and over 100 cemeteries were relocated during this time, including Dunbarton and Ellenton. Dunbarton was the town in which Duncannon was located; it was an early 19th-century plantation and a wildlife preserve. Former President George H. W. Bush and his brothers used to visit their grandfather George Herbert Walker at the plantation.[citation needed] Union General William T. Sherman allegedly spared the plantation, built in 1835, because a woman and sick child were resting in a bedroom upstairs.[citation needed]

U.S. Army soldiers were brought into the county and were used as guards at this new facility. A camp was constructed for the soldiers off Clinton Street, earning it the name "Barracks Road" among locals, in an area of the Little Salkehatchie swamp called O'Bannon Point. After discharge, many of these troops stayed on at SRP as civilian guards.[citation needed]

DuPont ran the Savannah River Site until 1989, when Westinghouse began the management of the facilities for the Department of Energy. The Savannah River Plant changed its name to the Savannah River Site. It was once one of the largest employers in the county.

The sundial is a unique vertical monument placed in front of the courthouse of Barnwell County. It is thought to be the only vertical freestanding sundial in the United States,[8] though counter-examples exist.[9] It was given to Barnwell in 1858 by Joseph D. Allen, at the time a state senator from Barnwell. The sundial was surrounded by a parking lot in the 1960s, but in the 1990s the city removed the parking lot, built a park, and made the sundial a focal point.[citation needed]

Bethlehem Baptist Church was officially organized in 1868, having its origin in the antebellum Barnwell Baptist Church, which was built on this site in 1829. The Barnwell Baptist Church had their services on this site until 1854 when the congregation built another church building on a different site in the town. At this time several free blacks and slaves were members of Barnwell Baptist Church, and they asked to use the 1829 sanctuary for worship services and meetings. Permission was granted to the group, and they met there informally until their official organization in 1868. The old Barnwell Baptist Church sanctuary continued to serve this newly organized church until the building was demolished in 1898. At the time it was demolished some of the material was salvaged and utilized in the present Bethlehem Baptist Church building.